1. Field of the Invention
The present invention relates to a method and an apparatus for recording data on a recordable disc, and more particularly, to a method and an apparatus for recording data on an existing recordable compact disc at a high density.
2. Description of the Related Art
Since video or audio data having large capacity are frequently transmitted and received over the Internet, there has arisen a need for a high-density recording medium with respect to which the data is easily recorded/reproduced. Recordable optical discs include CD-R/RWs having a capacity of 650 MB, DVD-RAM/R/RWs having a capacity of 4.7 GB, DVD+RWs having a capacity of 4.7 GB, and the like. Also, research on and the development of HD-DVDs having a recording capacity of more than 20 GB have been in progress. For DVDs, whereas DVD-ROMs are in widespread use by general users, DVD-RAM/R/RWs, DVD+RWs, and recording apparatuses capable of recording user data on DVD-RAM/R/RWs and DVD+RWs are not as widely accepted by general users.
However, CD-R/RWs are widely used as recordable optical discs. While data can be recorded only one time on CD-Rs, data can be repeatedly rewritten on CD-RWs. Further, only about 650 MB of user data can be recorded on CD-Rs and CD-RWs. Thus, Sony and Philips have developed and sold 1.3 GB double density compact discs (DDCD), which have a recording capacity that is nearly double that of the CD-R/RWs. FIGS. 1A and 1B are reference views for explaining differences between a DDCD and an existing CD. Referring to FIG. 1A, the DDCD has the same size as the existing CD shown in FIG. 1B. However, the DDCD has a track pitch of 1.1 μm, which is narrower than the track pitch of 1.6 μm of the existing CD. The DDCD has a minimum mark length (MML) of 0.623 μm, which is shorter than the MML of 0.833 μm of the CD. The numerical aperture (NA) of an optical pickup for the DDCD is 0.55 (reproducing/recording), which is about 10% greater than the NA of an optical pickup for the CD. The spot diameter of a laser beam for the DDCD is 1.17 μm, which is shorter than the spot diameter of 1.29 μm of a laser beam for the CD.
A simple method of increasing the recording density when changing the specifications of a pickup is to reduce the spot diameter of a laser beam for recording. Since the spot diameter is proportional to a wavelength λ of a light source and inversely proportional to the NA of an objective lens, the spot diameter can be reduced by (1) shortening the wavelength λ of the light source wavelength or (2) increasing the NA of the objective lens.
For the DDCD, the spot diameter can be theoretically reduced only by the second method. This is because it is highly possible to change parts of the pickup and the structure of the DDCD if the wavelength λ of the light source is changed. However, since in the DDCD case, the spot diameter reduces by 10% compared to the spot diameter of the CD, the resolution of a signal obtained from a recording mark becomes insufficient. Thus, a DDCE recording/reproducing apparatus additionally includes an equalizer to supplement the insufficient resolution.
The differences between a DDCD and a CD are summarized in Table 1.
TABLE 1DDCD-ROM/R/RWCD-ROM/R/RWRecording Capacity 1.3 GB  650 MBSpecifications of DiscDiameter 120 mm, Thickness 1.2 mmTrack Pitch 1.1 μm 1.6 μmMinimum Mark Length0.623 μm0.833 μmError Correcting SystemCIRC7CIRCModulation algorithmEFMWavelength of LightAbout 780 nmSourceNA of Objective Lens 0.5 (reproducing), 0.45 (reproducing), 0.55 (recording/ 0.50 (recording/reproducing)reproducing)Spot Diameter 1.17 μm 1.29 μm
Here, EFM denotes an Eight-to-Fourteen Modulation process, CIRC denotes a Cross Interleave Reed-Solomon Code, and the spot diameter is a diameter in which the intensity of a laser beam is equal to the central intensity 1/e2 (e is a natural great number).
Compared with the specifications of a CD, the specifications of a DDCD are as follows:                (1) The NA of the objective lens increases.        (2) The MML and the track pitch become short.        (3) An equalizer is added to the recording/reproducing apparatus.        (4) Efficiency of the error correcting system is strengthened.        
In other words, in the DDCD case, the spot diameter is shortened (specification (1)), the MML is reduced (specification (2)), and the resolution which is lowered through specification (1) is solved by increasing the efficiency of processing a signal through specification (4).
However, a tilt allowance (tilt margin) is reduced due to specification (1). Since the tilt allowance is inversely proportional to the cube of the NA, the tilt allowance reduces by about 25%. Also, the MML and the track pitch are reduced by 25% and 31%, respectively, through specification (2). However, since the spot diameter is reduced by about 10% through specification (1), the reduction ratios of the MML and the track pitch are greater than the reduction ratio of the spot diameter. Thus, there is an increase in cross erase or crosstalk between adjacent tracks and interference between adjacent marks when recording/reproducing data.
To solve these problems, when the allowable assembling error of a pickup device is reduced or a crosstalk canceller or a laser beam former is added, the manufacturing cost of the recording/reproducing apparatus increases more and more. Further, a user has to buy a new disc.